[Not Applicable]
1. Field of the Invention
The present invention relates to a golf club head. More specifically, the present invention relates to a polymer face section of a golf club head to reduce energy losses when impacting a golf ball.
2. Description of the Related Art
Technical innovation in the material, construction and performance of golf clubs has resulted in a variety of new products. The advent of metals as a structural material has largely replaced natural wood for wood-type golf club heads, and is but one example of this technical innovation resulting in a major change in the golf industry. In conjunction with such major changes are smaller scale refinements to likewise achieve dramatic results in golf club performance. For example, the metals comprising the structural elements of a golf club head have distinct requirements according to location in the golf club head. A sole or bottom section of the golf club head should be capable of withstanding high frictional forces for contacting the ground. A crown or top section should be lightweight to maintain a low center of gravity. A front or face of the golf club head should exhibit high strength and durability to withstand repeated impact with a golf ball. While various metals and composites are known for use in the face, several problems arise from the use of traditional face structure and materials. In addition, material interaction of the golf club head and the golf ball during impact is an important factor for performance of the golf club.
The golf ball is typically composed of a core-shell arrangement with a thin polymer shell, or cover material such as ionomers, surrounding a rubber-like core. These polymeric materials exhibit compression and shear, stiffness and strength properties dependent upon strain (load), input frequency (time dependency of small linear strain), strain rate (time rate of loading including large nonlinear strains), and temperature. The compression and shear stiffness properties of polymeric materials are measured and classified in terms of a storage moduli (Exe2x80x2, Gxe2x80x2) and a loss moduli (Exe2x80x3, Gxe2x80x3), respectively. The storage moduli (Exe2x80x2, Gxe2x80x2) represents the amount of compression and shear energy, respectively, stored during a complete loading cycle. For quasi-static loading, it is equivalent to the well known Young""s modulus (Exe2x80x2=E) and shear modulus (Gxe2x80x2=G=E /(2(1+xcexd)), where (xcexd) is the material Poisson ratio. For most polymers, the storage modulus increases significantly with strain, input frequency, and strain rate. For example, typical storage moduli for golf balls at low speed impacts, in the temperature range (50-100xc2x0 F.), are Exe2x80x2ball=450-6000 lb/in2 ball and Gxe2x80x2ball=150-2000 lb/in2. During high-speed impacts, in the temperature range (50-100xc2x0 F.), the typical storage are Exe2x80x2ball=9,000-50,000 lb/in2 and Gxe2x80x2ball=3,000-16,500 lb/in2. The low speed impact represents a putting stroke or a soft pitch shot, while the high-speed impact represents a golf swing with an iron-type or a wood-type golf club head.
The loss moduli (Exe2x80x3, Gxe2x80x3) represents the amount of compression and shear energy, respectively, dissipated during a cycle. For most polymers, the loss moduli also increase significantly with strain, input frequency, and strain rate, but the rate of increase can be very different than the aforementioned storage moduli. Finally, the magnitude of the loss moduli at a given strain, strain rate, frequency, or temperature typically vary from 0.005-2.0 times that of the storage moduli.
A loss (or damping) factor (xcex7E, xcex7G) or loss angle (xcex4E, xcex4G) for compression and shear are commonly defined as the ratio of the corresponding moduli;                                           η            E                    =                                    Tan              ⁢                              xe2x80x83                            ⁢                              δ                E                                      =                                          E                xe2x80x3                                            E                xe2x80x2                                                    ,                              η            G                    =                                    Tan              ⁢                              xe2x80x83                            ⁢                              δ                G                                      =                                                            G                  xe2x80x3                                                  G                  xe2x80x2                                            .                                                          [        t2        ]            
These loss factors are an important measure of the damping capability (energy loss mechanisms) of the material. For most ball-type materials, (xcex7Exe2x89xa1xcex7G) and magnitudes fall in the range of 0.005(low energy loss) to 2.0(high-energy losses), where magnitudes clearly depend upon polymer composition, strain, input frequency, strain rate, and temperature. As a comparison, the loss factors (energy loss mechanisms) in a metallic face of a golf club head are on the order of 10-100 times smaller than that of a golf ball. For most elastomeric polymer materials operating below the glass transition region, the Poisson ratio is fairly constant with (xcexd=0.4-0.5), while for stiff polymers acting at or above the glass transition region (xcexd=0.3-0.33).
Thus, during impact of the golf ball with the golf club head a significant portion of impact energy is lost as a result of the large deformations (0.05 to 0.50 inches) and deformation rates of the high damped golf ball materials, as opposed to the small deformations of the low damped metallic club face (0.025 to 0.050 inches) materials. A larger portion of this impact energy is lost in the golf ball because the magnitude of the deformation, the deformation rate, and energy loss mechanisms is greater for the golf ball than the face of the golf club head.
Application of hard polymers to the face of the golf club head represents a traditional structure of natural wood golf club heads, where a hard insert material centrally located in the face of the golf club and requiring an exacting fit between two or more distinct elements. The hard insert must be manufactured to a close tolerance to fit within a recess in the face of the golf club, and high surface hardness is less efficient in transferring energy to the golf ball during impact with the golf club. A homogeneous face structure is simpler to manufacture but is limited to the inherent material properties of the single material comprising the face structure. The present invention achieves a more efficient energy transfer during impact while maintaining a simple construction.
When a golf club head strikes a golf ball, large impact forces are produced that load the golf club head and the golf ball. Most of the energy is transferred from the golf club head to the golf ball; however, some energy is lost as a result of the impact. The present invention comprises an improved face structure for the golf club head to reduce impact energy losses, which could lead to greater efficiency in striking the golf ball. In a preferred embodiment the golf club head is a wood-type golf club head with a plurality of walls to define a hollow interior.
By allowing the golf club head to flex and cradle the golf ball during impact, the contact region as well as contact time between the golf ball and the face of the golf club head are increased, reducing the magnitude of the internal golf ball stresses as well as the rate of the stress build-up. This results in lower golf ball deformations and lower deformation rates to achieve lower energy losses in the golf ball during impact. The present invention accomplishes greater energy conserving impact by utilizing a specified polymer material layer on the face of the golf club head. During impact with the golf ball, the polymer layer compresses around the golf ball to enlarge the contact region and increase contact time of the golf ball, thus lowering the stresses and stress rate in the golf ball. Similarly, the polymer layer distributes the stresses to a backing structure in a more uniform manner. Also, the stress levels in the backing structure are significantly lower than the stresses of a similar metal golf club striking face without a polymer layer because there are no scorelines in the backing structure which serve to amplify the stresses. Thus the backing structure can be made thinner, and more flexible, than typical existing metal wood-type golf club heads. The more flexible backing structure coupled to the polymer layer can lead to even lower energy impact losses. The golf club head may be constructed from rigid material and still obtain the benefits of the present invention.
Coefficient of restitution (COR) is well known to those of ordinary skill in the art, and is defined as the ratio of the relative velocity of the golf ball to golf club head just after impact divided by the relative velocity of the golf head to golf ball just before impact. Expressed mathematically, the equation is outlined below                     COR        =                                            V                              2                -                Ball                                      -                          V                              2                -                Head                                                                        V                              1                -                Head                                      -                          V                              1                -                Ball                                                                        [        t1        ]            
where V2-Ball is the velocity of the golf ball measured immediately after impact with the golf club head; V1-Ball is the velocity of the golf ball measured immediately before impact with the golf club head; V1-Head is the velocity of the golf club head measured immediately before impact with the golf ball; V2-Head is the velocity of the golf club head measured immediately after impact with the golf ball.
Polymer material chemistry and thickness determines important performance variables including durability, coefficient of restitution (COR) and material stress levels. In a preferred embodiment the polymer material should have a lower nominal (quasi-static) storage compression (Exe2x80x2) and storage shear (Gxe2x80x2) moduli, lower nominal loss compression (Exe2x80x3) and loss shear (Gxe2x80x3) moduli, and damping properties (xcex7E, xcex7G) than these same properties of the golf ball (Exe2x80x2ball, Gxe2x80x2ball, Exe2x80x3ball, Gxe2x80x3ball, and (xcex7E, xcex7G)ball, respectively. Thus the polymer layer on the face of the golf club head will deform around, or cradle, the golf ball with lower energy loss mechanisms than the cover material of the golf ball. Since these polymer material storage and loss moduli significantly increase with golf club head impact speed, an important goal for the polymer material on the face of the golf club head is to have an effective lower storage compression and loss compression moduli (Exe2x80x2, Exe2x80x3) and storage shear and loss shear moduli (Gxe2x80x2, Gxe2x80x3) than the golf ball (Exe2x80x2ball, Gxe2x80x2ball, Exe2x80x3ball, Gxe2x80x3ball) at the higher loading rates and input frequency found in the high speed impact associated with the wood-type golf club head. These loading rates are typically 1000-5000 in/in/sec and the input frequency is typically 500-4000 cycles/sec. Thus, polymer face materials that have higher storage and loss moduli than the golf ball at low load rates are also covered by the present invention, as long as the polymer face materials have lower effective storage and loss moduli than the golf ball at wood-type golf club head impact load rates. Ideally, the storage and loss moduli of the polymer face material would be lower than the golf ball properties and be strain, strain rate, and input frequency insensitive. Performance benefits can be obtained when the polymer face material has a storage and/or loss moduli limit of about twice the storage and/or loss moduli of the ball material. The polymer utilized in the face of the golf club head is much softer than a typical metallic face and the impact duration between the golf ball and the golf club head is increased.
One object of the present invention is to improve impact efficiency between a golf club head and the golf ball.
Another object is to incorporate a polymer material in the face section of a golf club head to perform as a compliant golf club face. Any number of rigid materials can be utilized in the manufacture of the golf club of the present invention to produce a compliant, or softer flexing performance, golf club face during impact with the golf ball.
A further object of the present invention is a wood-type golf club head having a Polymer face material with a storage compression and storage shear modulus less than that of a golf ball at low loading rates.
Another object of the present invention is a wood-type golf club head having a polymer face material with a storage compression and storage shear modulus less than that of a golf ball at high loading rates.
Another object of the present invention is a wood-type golf club head having a polymer face material with a loss compression and loss shear modulus less than that of a golf ball at low loading rates.
Another object of the present invention is a wood-type golf club head having a polymer face material with a loss compression and storage shear modulus less than that of a golf ball at high loading rates.
Another object of the present invention is a wood-type golf club head having a polymer face material with a storage compression and storage shear modulus less than or equal to double the storage compression and storage shear moduli of a golf ball at low loading rates.
Another object of the present invention is a wood-type golf club head having a polymer face material with a storage compression and storage shear modulus less than or equal to double the storage compression and storage shear moduli of a golf ball at high loading rates.
Another object of the present invention is a wood-type golf club head having a polymer face material with a loss compression and loss shear modulus less than or equal to double the loss compression and loss shear moduli of a golf ball at low loading rates.
Another object of the present invention is a wood-type golf club head having a polymer face material with a loss compression and loss shear modulus less than or equal to double the loss compression and loss shear moduli of a golf ball at high loading rates.
Another object of the present invention is a wood-type golf club head having a face insert supported by a polymer material for flexing of the golf club face.
Another object of the present invention is to have a polymer face material composed of multiple polymer and/or metallic layers where the outer layers may be designed for improved durability and or spin-control.
Another object of the present invention is to have a nonhomogeneous polymer face material so that different material formulations may exist over the polymer face for the purpose of increasing impact velocity for center shots, but lower velocity or controlled spin for off-center shots.
Another object of the present invention is to have a golf club head with scorelines in a polymer face material.
Having briefly described the present invention, the above and further objects, features and advantages thereof will be recognized by those skilled in the pertinent art from the following detailed description of the invention when taken in conjunction with the accompanying drawings.